A mathematical model has been developed to describe the interaction of peptide hormones with cells. It considers a two step binding at the surface, interalization through coated pits, lysosomal degradation of hormone and possible recycling of the receptors. The model has been applied to a wide variety of kinetic data on EGF with excellent agreement between theory and experiment. Equilibrium studies lead to linear or curviniear Scatchard plots depending on the model parameters and suggests that the saturation of coated pit proteins rather than receptor heterogeneity could be responsible for experimental concave plots. Furthermore, the interpretation of slopes and intersection in terms of affinities and number of receptors is not immediate if internatlization occurs. We are developing a model for the release of histamine following the binding of IGE to basopohils. The model accounts for the kinetics of several biochemical events including methylation of phospholipids and the release of arachidonic acid. These reactions are linked to the influx of calcium into the cell which was also incorporated into the model. The model predicts the time course of these responses, as well as those regulatory membrane enzymes, and the release of histamine from the cell. Data from mutant strains have been incorporated, while data on sensitization and desenitization have yet to be modeled. This system has features which are common to many excretory pathways, such as hormone release, and the model should therefore be generally useful of exocytosis of compounds from cells.